The present invention relates generally to the thickness reduction of metal slabs, and particularly to a method of reducing the thickness of slabs in a manner that avoids alligatoring in the ends of the slabs.
Large ingot that is slated for reduction in thickness for purposes of being rolled in a rolling mill to make sheet or plate products typically requires "breakdown" in a rolling mill. Breakdown rolling involves numerous passes of the ingot through the mill, usually a reversing mill, at temperatures in the range of about 250.degree. C. to about 550.degree. C. for aluminum ingot. This provides a slab of metal material. After appreciable amounts of reduction involving such passes of the slab through the mill, fractures tend to occur in the ends of the slab. These fractures extend in a central plane that is parallel to the rolled surfaces of the slab.
Such fracturing, or alligatoring, as it is known in the rolling art, may be caused by an internal stress state in the slab which is the result of nonuniform deformations that take place in the initial passes. As the breakdown rolling process proceeds, internal stresses may be sufficient to open up a slab that contains central defects or is made of a material of limited ductility, as in the case of a slab of the aluminum 8% magnesium alloy.
Whatever the cause of alligatoring, the subject matter of the present invention is directed to substantially limiting the occurrence of such alligatoring, if not preventing the same altogether. It can be appreciated that with the occurrence of alligatoring a substantial amount of scrap is generated, as the split portions of the slab must be removed from the main body thereof before the slab is further reduced. The removal of the split ends also increases costs by increasing the breakdown rolling time for each ingot, i.e., there is lower productivity. Control or elimination of alligator formations therefore reduces scrap loss and rolling time, and thus improves the efficiency of the breakdown rolling process.